SYDNEY (AP)—Former U.S. Open champions Juan Martin del Potro and Samantha Stosur rallied to win their second-round matches against tough French opponents Wednesday at Australian Open warm-up tournaments.

Stosur lost all three of her singles matches at the Hopman Cup team exhibition event last week and is looking for some confidence heading into her home Grand Slam at Melbourne Park, where she has never advanced past the fourth round.

''Any win's a good win, but when you've got to really tough it out and battle through a long match ... you're probably holding your nerve pretty well,'' Stosur said.

The Australian Open begins Monday.

Earlier, former Wimbledon champion Petra Kvitova advanced to the semifinals of the women's event in Sydney with a 7-6 (4), 6-2 victory over Lucie Safarova. Kvitova, the second seeded player, will play Tsvetana Pironkova in the semifinals after the Bulgarian's 7-6 (2), 6-3 win over Sara Errani.

In a late match, Madison Keys advanced to the semifinals when Bethanie Mattek-Sands retired with a lower back injury while trailing 3-2 in the first set. Mattek-Sands, who beat top-seeded Agnieszka Radwanska on Tuesday, said she still hoped to play in the Australian Open.

David Ferrer, the No. 1-seeded player and defending champion, beat Donald Young of the United States 6-7 (5), 6-3, 6-1. The Spaniard is seeking his fifth Auckland singles title.

Third-seeded John Isner also progressed to the quarterfinals with a 7-6 (1), 3-6, 6-2 win over Lucas Lacko. Isner's participation in the tournament had been in doubt because of an ankle injury, but the American seemed fit in a match that lasted nearly two hours.

American qualifier Steve Johnson beat Marcos Baghdatis 7-5, 6-7 (3), 6-4 in a first-round match held over from Tuesday because of rain.